Various surface pretreatments have been practiced with metallic substrates as iron, zinc plated steel, aluminium and other plates, to improve corrosion resistance and coating adhesion thereof. Among the treatments proposed, particular attention is directed to, from the standpoint of easiness in operation and operational control, obviation of drain contamination, shortening of process steps and the like, a metal surface treatment with the so-called coating type, non-rinse chromate treating agent, wherein the treating liquid containing, as the main ingredients, a synthetic latex and a water soluble chromium compound is merely applied to the metal surface.
For this end, a number of proposals have been made as, for example, Japanese patent publication No. 31026/74; ibid 40865/74; ibid 1889/75 and the like. However, in the heretofore proposed treating compositions, troubles have always been encountered due to the presence of a surfactant or emulsifier used for the preparation of an emulsion. That is, for the purpose of maintaining an emulsion in its stabilized state the, employment of a surfactant or a emulsifier is essential but the presence of such material in the formed emulsion may inevitably cause adverse effects on the adhesion the, anticorrosion, and the water resistance of the coating prepared therefrom. Therefore, efforts have been made to solve the question of chemical stability of the emulsion without the help of conventional type surfactants or emulsifiers and obtain a stabilized composition comprising such emulsion and a water soluble chromium compound containing a trivalent and hexavalent Cr.
In the meantime, a technique has been proposed of using a water soluble type organic high molecular compound as an emulsifier in the preparation of a resinous emulsion. That is, in Japanese patent application Kokai No. 74934/76, there describes a composition comprising a polymeric emulsion and chromium compound, which is characterized by that the emulsion is prepared by using a defined amounts of particular polyacrylic acid or its ammonium salt and polymerizing .alpha.,.beta.-ethylenically unsaturated monomers, in the presence of water soluble persulfate, at a defined temperature. Also, in Japanese patent publication No. 39393/81, there is disclosed a metal surface treatment with the composition comprising as principal ingredients, a polymeric emulsion, a water soluble chromium compound and a water insoluble white carbon, the emulsion being prepared by emulsion polymerization of .alpha.,.beta.-monoethylenically unsaturated monomers using, as an emulsifier, specified amounts of polyacrylic acid and/or acrylic copolymer. The thus obtained emulsion per se, i.e. the emulsion obtained by using a water soluble organic high molecular compound as an emulsifier, is quite stable chemically and can give a stabilized composition when compounded with a chromium compound containing trivalent and hexavalent chromium and the resulting composition is very useful as a metal surface pretreating composition capable of forming a pretreatment film with excellent corrosion resistance and adhesion properties. However, in considering the surface pretreatment of the metallic substrate, it is of great importance to take into account, besides the stationary adhesion between the substrate and the composition, the coating adhesion under bending and processing conditions, as well as the scratch resistance of the coating. Generally speaking, the properties of film adhesion under bending conditions and of scratch resistance conflict with each other. In the aforesaid Japanese patent application Kokai No. 74934/76, attention is only directed to the adhesion under processing conditions and not to the scratch resistance at all. And, in Japanese patent publication No. 39393/81, the claimed effects are merely of the interaction of the disclosed three components, i.e. the emulsion, the water soluble chromium compound and the water insoluble white carbon, and no statements are given as to the scratch resistance and bending behavior in the two component system as in the present invention. Furthermore, even in the said three component system, the effects of scratch resistance and bending resistance fluctuate considerably and it was found that good results were not always obtained therewith. Especially, a low temperature bending will often cause troublesome adhesion failure and there is a trend that the more the trivalent chromium compound, the lesser the storage stability of the liquid concentrate.
Accordingly, an object of the present invention is to provide a metal surface pretreating composition comprising as principal ingredients, a polymeric emulsion and a chromium compound, the emulsion per se being stable without the help of conventional type surfactants or emulsifiers and capable of prducing a stabilized composition when compounded with a chromium compound, and the thus obtained composition having, especially prior to dilution with water, good storage stability and being capable of forming a coating with an excellent combination of properties of corrosion resistance, water resistance, scratch resistance and bending performance at normal and low temperature and the like. The invention is on the same line with those of the above said Japanese patent application Kokai No. 74934/76 and Japanese Patent Publication No. 39393/81 in that the emulsion stability and chemical stability can be maintained in the absence of conventional surfactants or emulsifiers and an excellent pretreatment coating in regard to corrosion resistance, water resistance and coating adhesion can be obtained therefrom. The invention has been made to achieve the above characteristics while paying attention to the stress relaxation in the metal surface treatment to obtain a good balance of each of the conflicting requirements of bending resistance and scratch resistance. Recently, a precoat metal plate, has been placed on the market, which is hardly ever or never cracked during the bending and processing thereof. In such a product, stress relaxation by cracking is primarily eliminated and therefore coating adhesion naturally become deteriorated with such a uniform coating. However, if something be devised to obtain an uneven coating wherein particle joining points are intentionally scattered throughout the coating, it would be quite effective for the aforesaid stress relaxation. Metal surfaces are, from the very nature of things, very irregular microscopically. Therefore, if the resinous particles are made in sufficiently smaller size so as to enter into the depressed portion and of hard nature, resistance to shearing stress would be naturally increased, thereby promoting stress relaxation in the proximity of the particles. With the abovesaid in minds, the inventors have devoted themselves to series of works and found that a coating type, non-rinse chromate pretreating composition capable or forming an excellent pretreatment in regard to corrosion resistance, bending and processing resistance, scratch resistance and the like can be obtained with the emulsion whose constituting polymer microparticles are of defined mean diameter and of hard material with the defined range of glass transition point or with the internally gelated structure, and on the basis of this finding, have completed the invention.